1. Field of the Invention
The present invention relates to in-line fluid-type filters for use in automated clinical instrumentation, and more particularly to an in-line fluid-type filter which may be advantageously used to filter whole blood.
2. Prior Art
Clinical apparatus for the analysis of fluids are well known. See, for example, U.S. Pat. Nos. 2,797,149, 2,879,141 and 3,241,432. Typically, analysis apparatus of the automated type provide for the feeding of samples in a flowing stream by means of a take-off device or probe which aspirates liquid sample from a sample container. The aspirated portion of liquid sample is then conveyed through suitable conduits for analysis.
In present day analysis apparatus, only a small quantity of sample may be employed for analysis purposes. Typically, sample flow in the take-off device is at a relatively slow rate, e.g. 1.5 ml/min. The sample conduit and the conduits employed in such apparatus extending from the take-off device to the point of analysis must be relatively small. By way of example only, the sample conveying conduit may have an internal diameter of approximately 0.02 to 0.033 inch. Such a conduit may become clogged during the performance of a series of tests and necessitate the shutdown of the system to clear the conduit. Such clogging or other interference from debris may be the result of foreign matter in the sample or may be due to the existence in the sample of a naturally occurring substance, such as, for example, fibrin, that the substance in whole blood which acts to form the fibrous network in the coagulation of blood.
The prior art has attempted to solve the clogging problem by proposing diverse filtering schemes, but this is not as simple a solution as might first appear. In any contemplated device designed to filter the sample to remove such potential clog-causing matter, care must be taken to avoid restricting significantly the flow of the sample through the filtering device. In view of the relatively small sample size, the amount of sample retained in the filtering device must be kept to a minimum. It is also important when dealing with filtering of whole blood samples that the laminar flow within the conduit be disturbed as little as possible, and that there be a smooth transition downstream from the filter so as not to disrupt the integrity of, e.g. rupture, the cells within the sample.
Filters may be provided at various locations in the conduit systems to catch the clogging or interference causing debris. For example, there are disclosed in U.S. Pat. No. 3,795,149, a method and apparatus for supplying samples for automated analysis wherein liquid from a liquid sample container is aspirated through a filter-equipped inlet end of a probe while the latter is immersed in the liquid. The probe is subsequently removed from the container and immersed in the liquid of a wash receptacle. Prior to immersion in another liquid sample, a fluid other than sample is flushed through the aforementioned filter in a reverse direction to cleanse it of particulate matter, the flushing being in timed relation to the movements of the probe. More particularly, the probe includes a cup-shaped filter extending over the inlet end of a take-off tube. The filter may be formed from a disc of stainless steel, for example, which is suitably etched to provide filter holes therethrough and which is bent up to provide the cup shape. The filter surrounds the inlet in a manner to provide a filtering action. It has been found, however, that cup-shaped filters such as that just described require a significantly high pressure on backflushing to dislodge the material caught in the filter. Although filters formed of material such as stainless steel may appear to the eye to be smooth, nevertheless, there are sharp protuberances and burrs on the surfaces which can trap fibrous matter so that it is not easily dislodged from the filter by backflushing under normal flow pressure, necessitating increased flow pressure for the backflushing cycle. Also, as will be noted hereinbelow, such fibrous matter often becomes further entwined in the filter on backflushing, leading to permanent clogging of the filter after a relatively few cycles. This leads to a high frequency of instrument down time required to clean and/or replace the filter. Furthermore, filters of the type disclosed in U.S. Pat. No. 3,795,149 are preferably fixedly secured to the inlet of the take-off tube, requiring the replacement of the take-off tube assembly each time the filter needs to be replaced.
The use of disc or wafer-shaped filters in an in-line fluid filter arrangement is quite common. In U.S. Pat. No. 4,263,140, there is disclosed such an arrangement which includes a pair of body sections coaxially secured to each other. A filter element is fixedly disposed transversely across a filter chamber defined by the body section intermediate a fluid filter inlet and outlet. This filter element includes an annular mounting flange interposed between mating annular body section end faces. A generally cup-shaped filter element support is fixedly located on at least the outlet side of the filter element and is dimensioned so that the filter element is at least partially received in the cup-shaped area thereof. This support includes an annular mounting rim which is also interposed between the body section end faces. The support allows the filter to experience greatly increased fluid pressure differentials across the filter element. The filter element mounting flange and the support mounting rim are dimensioned to at least extend to the outside diameter of the mating body section annular end faces. The body sections are rigidly affixed to each other at the end faces by means of a fusion type weld with at least a portion of the filter element mounting flange and the support mounting rim comprising a filler material for the weld to assist in producing a joint of high integrity. A pair of the filter element supports may be advantageously employed wherein the supports are in an opposed relationship to each other having the filter element positioned therebetween.
While in-line disc-type filters may prove effective in restricting the passage of particulate foreign matter through the conduit in which they are placed, it has been found that such filters, particularly when used in clinical apparatus for the analysis of whole blood, readily become clogged by the fibrin in the sample. As the aspirated sample of whole blood passes through the filter, the fibrous matter becomes trapped by the filter as intended, but problems often arise in cleaning the trapped matter from the filter. The fibrous matter generally has a length much greater than the width of the filtering element. Attempts to clean the filter by backflushing usually result in the fibrous strand becoming further entangled in the filter, and permanently lodged therein, clogging one or more passages in the filter. The filter is usually sized so that the entanglement of a few fibrous strands will not significantly effect fluid flow therethrough; however, it will be readily appreciated that the filter will eventually become so clogged by the fibrous matter that it seriously restricts fluid flow therethrough, and cannot be cleaned by simple backflushing. It must be replaced. Replacing such in-line filters requires shutting down the apparatus, thus interfering with sample analysis.
A representative sampling of other prior art filtering arrangements intended for filtering blood and other body fluids includes U.S. Pat. Nos. 3,493,503, 3,882,026, 4,170,056, 4,157,967, 4,476,023 and 4,370,381, each of which describes a disc-type filter.
While the prior art demonstrates the development of filter arrangements for automated clinical analyzers to the best of our knowledge, the prior art does not teach or describe a filter arrangement which will effectively remove potential clog causing materials from the sample fluid stream while at the same time lengthening the interval between filter backflushing and replacements. It is a principal object of the present invention to provide such a filter.